The present invention relates to electret condenser microphones, and, more particularly, to a system and method for improved RF immunity of an electret condenser microphone.
Electret condenser microphones find application in a myriad of applications. For example, speakerphones and telephone handsets often use electret condenser microphones. Speakerphones generally are a high gain system that may use small electret condenser microphones and use radio frequency (“RF”) technology to transmit and receive voice data. Handsets often use lower microphone gain as the microphone is close to the desired audio source, i.e., a voice.
The RF technology often used in systems including electret condenser microphones are common RF modulation schemes, such as the global system for mobile communication (“GSM”) standard, the digital enhanced cordless telecommunications (“DECT”) standard, cellular standards, or the time division multiple access (“TDMA”) standard. These common schemes generally send data in bursts or packets over microwave frequency bands. The RF signal is demodulated at the microphone's junction field effect transistor (“JFET”) internal amplifier. This demodulation corrupts the audio output signal with objectionable noise.
More specifically, in these transmission schemes, the carrier transmits the audio in the form a digital burst or data packet that is compressed in time. Such compression causes the RF energy to be transmitted in bursts. Demodulation, by the PN junction of the JFET amplifier in the microphone of the energy bursts, can result in audible interference, and, hence, corruption of the audio output signal. Typical transmission rates for these time multiplexed packets are on the order of 5, 10, or 20 milliseconds of voice data transmitted in a much shorter duration packet each 5, 10, or 20 millisecond period.
The microphone cable, i.e., the connecting cable connected to the terminals of the microphone and the system of which the microphone is a part, normally acts like an antenna. Thus, the microphone cable picks up some of the RF energy being transmitted by the system that contains the microphone, and then conducts the picked up RF energy to the microphone terminals.
Prior art systems may be useful in applications where microphone gain is not too high, such as handset use where the microphone is close to the mouth) and does not require much amplification of the microphone's output signal. In cases, such as wireless speakerphones, that require much higher microphone gains, in addition to the placement of capacitors, are generally not effective and, thus, the microphone element usually must be shielded by using ferrite beads, conductive tape, or other methods to reduce the RF energy seen by the JFET amplifier. Such shielding assists in preservation of a reasonable signal to noise ratio, but adds significant expense to the manufacture of such electret condenser microphones.
It is desired to provide a system and method with improved RF immunity for an electret condenser microphone. It is also desired for such a system and method to be effective in systems using common RF transmission schemes. It is further desired to provide a system and method that does not introduce significant cost to the manufacture of a system including at least one electret condenser microphone.